The invention relates to a method for operating a conveyor installation comprising a belt and at least one drive drum for driving said belt.
Conveyor installations, by way of example trough conveyors or tubular belt conveyors, include a belt, which is used to convey material to be conveyed, and at least one drive drum, with the aid of which the belt can be driven. Conveyor installations also include deflection drums and a drive system for driving the drive drum. This drive system in turn includes a gear and a motor. One or more frequency converter(s) is/are used for driving and controlling the motor or motors in particular in the case of relatively long conveyor belts.
If the speed of the belt is to be altered, by way of example when starting up or stopping the conveyor installation, then the aim is to bring the belt from one speed level to another speed level, i.e. to decelerate or accelerate the drive drum from a first rotational speed to a second rotational speed, so as to be as free from tension as possible. It is desirable in particular when stopping the conveyor installation to tension the belt as little as possible. Operation of the conveyor installation in this way spares the belt and then also simplifies restarting.
To alter the speed of the belt, rotational speeds and the timings thereof with which the motor and the drive drum driven thereby should rotate, are specified by the frequency converter. The course of a corresponding rotational speed characteristic curve is smoothed and adjusted by a ramp function generator such that only low torsional or force impacts act on the material, i.e. in particular the drive drum and belt. The run-up and run-back times and ramp rounding times of the rotational speed characteristic curve for jerk limitation can be given for this purpose. Ramp rounding of the rotational speed characteristic curve is taken to mean that the rotational speeds of the drive drum alter steadily rather than suddenly. During an acceleration process by way of example therefore the rotational speed characteristic curve does not have a true ramp shape and is instead rounded at the start and at the end, so it has an S curve. Despite such a design of the rotational speed characteristic curve the belt is undesirably tensioned in the event of an alteration in speed. The torque increases massively just before the belt comes to a standstill and up to the time when the converter is switched off, in particular when the conveyor installation is stopped. The torque is suddenly set at zero at the switch-off time. The preceding increase in torque only tensions the belt, however, and does not have any positive effect on the stopping process. After the converter has been switched off the belt springs back as a result of its tensioning and the speed gains appreciable momentum in the negative direction. The belt relaxes slightly as a result of this springing-back. The run-back and ramp rounding times have previously been adjusted to avoid tensioning of this kind and the subsequent springing-back of the belt. However, this also leads to only a slight improvement.
A method for controlling the start-up of a conveyor installation comprising a belt and at least one drive drum for driving the belt is known from WO 01/53174 A1, in which the desired value of the speed of the belt is increased in a first period with increasing acceleration and in a second period with decreasing acceleration. The effects mentioned above also occur as a result of such an alteration in the speed, however.